Magnetic resonance imaging (MRI) systems require a uniform magnetic field and radio frequency radiation to cause magnetic resonance in the atomic nuclei of the subject being imaged. The magnetic resonance of the nuclei provides information from which an image of the portion of the subject containing these nuclei may be constructed. An exemplary method of MR imaging may be found in U.S. Pat. No. 4,471,306, assigned to the assignee of the present invention.
The magnetic field must be highly homogeneous, e.g. it should not vary more than several milligauss (1 gauss=10.sup.-4 tesla) per centimeter, in order to obtain a meaningful image of the subject. Presently, both permanent magnets and superconducting magnets are used for generating such field. Among the advantages of permanent magnets are lower cost and a magnetic field which steeply drops off to near zero in the area outside of the magnet as distance from the magnet increases. The use of a permanent magnet instead of a superconducting magnet also eliminates the liquid helium needed to maintain the low temperature of a superconducting magnet.
Although permanent magnets allow realization of a cost savings over superconducting magnets, the permanent magnet materials used are expensive. In addition, the permanent magnets are very heavy due to the amount of material needed to provide the uniform magnetic field and to provide a flux return path within the permanent magnet volume. Present permanent magnet assemblies for MRI frequently require structural reinforcement in the building where they are installed due to their large mass.